calcimycin and methylmercuric-chloride

Methylmercury chloride (MeHgCl) is well known as a significant environmental hazard, particularly as a modulator of the immune system. As it is acknowledged that the critical effector cells in the host response participating in various biological responses are mast cells, we tried to define the possible contribution of mast cells in the development of methylmercury-evoked effects. We investigated the effects of methylmercury on the rat mast cell degranulation induced by non-immunological stimuli (the selective liberator of histamine, compound 48/80, and calcium ionophore A23187) both in vivo and in vitro. Using the cells prepared from methylmercury-intoxicated rats through a 5-day treatment of MeHgCl (10 mg/kg/day), we observed the suppression of calcium ionophore A23187- and 48/80-induced histamine release, which was enhanced with time after treatment. Similar suppression was observed in the ionophore-stimulated release, when cells were prepared from rat with a single treatment of MeHgCl (20 mg/kg). It should be noted that when cells from the control rat were pre-incubated with methylmercury in vitro at a 10(-8) M concentration for 10 min, A23187 and compound 48/80-stimulated histamine release was significantly enhanced. However, when the pre-incubation period was prolonged to 30 min, the release was suppressed. An increase in the methylmercury concentration to 10(-6) M also suppressed the histamine release. These results show that methylmercury treatment can modify mast cell function depending on concentration and time, and might provide an insight into the role of mast cells in the development of methylmercury-stimulated effects.

Topics: Adjuvants, Immunologic; Administration, Oral; Animals; Calcimycin; Cell Degranulation; Cells, Cultured; Dose-Response Relationship, Drug; Drug Interactions; Histamine Release; Male; Mast Cells; Methylmercury Compounds; p-Methoxy-N-methylphenethylamine; Rats; Rats, Wistar

In an effort to explain the previously observed methyl mercury (MeHg)-induced stimulation of protein phosphorylation in cerebellar granule neuron cultures, the effect of MeHg on protein kinase activities in cell-free assays and on second messenger systems in cultured neurons has been examined. Using cell-free assays for several protein kinases, no stimulation of enzyme activity was found at any concentration of MeHg tested. After 24 h exposure, 1-5 microM MeHg was found to have no significant effect on neuronal cyclic AMP levels. In contrast, intracellular levels of Ca2+ and rates of 45Ca2+ uptake were elevated 2.2-fold and 3.6-fold, respectively, by 5 microM MeHg. These effects were not observed with mercuric chloride, triethyllead, or lead acetate. Measurement of inositol phosphate production in granule cell cultures revealed a sensitive, pretoxic effect of MeHg with twofold stimulation following 30-min exposure to 5 microM MeHg and 1.6-fold after 24-h exposure to 3 microM MeHg. Detection of inositol phosphate production after 30 min of MeHg was largely neuron-specific. These results suggest that second messenger-mediated activation of select protein kinase enzymes may be the mechanism underlying MeHg-induced stimulation of protein phosphorylation in cerebellar neuronal culture. In addition, these findings indicate a specific interference with neuronal signal transduction and suggest a basis for the selective neurotoxic action of this agent.

Topics: Animals; Calcimycin; Calcium; Calcium Radioisotopes; Cells, Cultured; Cerebellum; Cyclic AMP; Inositol Phosphates; Methylmercury Compounds; Neurons; Phosphorylation; Protein Kinase Inhibitors; Protein Kinases; Rats; Rats, Sprague-Dawley; Second Messenger Systems

Icosanoid formation in platelets depends on the concentration of free arachidonate that is mainly liberated from membrane phospholipids by phospholipase A2. The concentration of free arachidonate is also controlled by the activities of the reacylating enzymes arachidonoyl-CoA synthetase and lysophospholipid acyltransferase. In human platelet microsomes we determined the high enzyme activities of 5.9 nmol.min-1.(10(9) platelets)-1 for the arachidonoyl-CoA synthetase and 37 nmol.min-1.(10(9) platelets)-1 for the lysophospholipid acyltransferase. The activities of these reacylating enzymes were strongly reduced by hydrogen peroxide (H2O2) and methyl mercury that are primary stimuli of arachidonate release in intact platelets. H2O2 inhibited the arachidonoyl-CoA synthetase with an IC50 of 3.3 mmol/l without affecting the lysophospholipid acyltransferase. Sulfhydryl group protection by 3-mercapto-1,2-propanediol did not overcome the inhibition but glutathione prevented the inhibition of the arachidonoyl-CoA synthetase by H2O2. This suggests that glutathione by virtue of the glutathione peroxidase reduces H2O2 rather than that it protects free sulfhydryl groups of the arachidonoyl-CoA synthetase. Methyl mercury left the arachidonoyl-CoA synthetase activity unaffected but inhibited the lysophospholipid acyltransferase activity with an IC50 of 3.4 mumol/l. The inhibition is probably evoked by the blockade of sulfhydryl groups of the lysophospholipid acyltransferase because it disappeared when 3-mercapto-1,2-propanediol was added at a concentration higher than that of methyl mercury. Thrombin as a physiological full agonist, Ca2+ less than or equal to 1 mmol/l, the calcium ionophore A23187 and phorbol 12-myristate 13-acetate (TPA) and 1-oleoyl-2-acetylglycerol as model stimuli of protein kinase C neither influenced arachidonoyl-CoA synthetase nor lysophospholipid acyltransferase. It is concluded that the inhibitory effect of H2O2 and methyl mercury on the arachidonate-reacylating enzymes arachidonoyl-CoA synthetase or lysophospholipid acyltransferase, respectively, are responsible for their capacity to stimulate icosanoid release in intact cells. Thrombin and its intracellular messengers Ca2+ and diacylglycerol do not directly affect arachidonoyl-CoA synthetase and lysophospholipid acyltransferase.

Topics: Acyltransferases; Blood Platelets; Calcimycin; Calcium; Coenzyme A Ligases; Diglycerides; Eicosanoids; Glutathione; Humans; Hydrogen Peroxide; Kinetics; Lysophospholipase; Methylmercury Compounds; Microsomes; Multienzyme Complexes; Phospholipases; Tetradecanoylphorbol Acetate; Thrombin